The electrolyte solution is a crucial component in an ambient temperature secondary lithium cell. A non-aqueous solvent or mixture of solvents which dissolves an appreciable amount of lithium salts to form highly conducting solutions is desirable. The electrolyte should afford high efficiency for cycling of the lithium or lithium-ion electrode, and exhibit good thermal stability up to 70.degree. C. (the usual upper temperature limit for operation of ambient temperature batteries).
A highly desirable liquid electrolyte solution established for ambient temperature Li secondary cells is described in U.S. Pat. No. 4,489,145. It comprises a solution of LiAsF.sub.6 dissolved in a mixed solvent of tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-Me-THF), and 2-methylfuran (2-Me-F). Other aprotic electrolytes have contained cyclic carbonates such as propylene carbonate (PC) and ethylene carbonate (EC), and have been the subject of much study in terms of both basic electrochemistry and battery applications for many years.
More recent developments have included the use of electrolytes based on polymers such as polyacrylonitrile (PAN), poly(tetraethylene glycol diacrylate) (PEGDA), poly(vinyl) pyrrolidinone (PVP), poly (vinyl chloride) (PVC), or poly(vinyl sulfone) (PVS). In these electrolytes the polymers are matrices to immobilize complexes (solvates) formed between Li salts, such as LiAsF.sub.6, LiCF.sub.3 SO.sub.3, LiPF.sub.6, LiN(CF.sub.3 SO.sub.2).sub.2 and LiBF.sub.4, and an aprotic organic solvent (or mixture of such solvents) to allow fabrication of free-standing electrolyte films to be used in solid-state Li and Li-ion batteries (K. M. Abraham et al., U.S. Pat. Nos. 5,219,679; 5,252,413; 5,457,860).
Unlike aqueous cells, non-aqueous electrolyte (organic electrolyte) cells may not be overcharged without causing irreversible electrolyte side-reactions which deteriorate cell performance. Cells are safeguarded during laboratory charge/discharge tests by careful control of the voltage limits by means of the electronic equipment used in the test. Electronic overcharge control comprises a sensing circuit which prevents current from flowing into the cell once it reaches the voltage corresponding to complete charge. The charge voltage limit is selected according to the electrochemical couple in the cell. For example, Carbon/LiMn.sub.2 O.sub.4 cells have an upper charge limit of 4.3V vs. Li.sup.+ /Li.
Chemical overcharge protection of a battery consisting of cells connected in series is particularly important for two reasons. Firstly, it will replace electronic overcharge controllers in individual cells. Electronic controllers lower the energy density of the battery and increase battery cost. Secondly, it will provide capacity balance among the individual cells and prevent oxidative degradation of the electrolyte. The capacity balance among the cells in a battery may be lost, especially after repeated charge/discharge cycles. This means that the accessible capacity of individual cells may not remain equal. In this instance, the cathode of the cell with the lowest capacity will be pushed above the normal upper voltage limit. Oxidative degradation of the electrolyte will occur at these higher potentials, and this will degrade the cycle life of the battery at an accelerated rate. Even if the electrolyte does not decompose, the weaker cell will contribute a larger fraction of the total cutoff voltage for the battery causing the capacity of the cells in the battery to become increasingly out of balance at each additional cycle, since the stronger cells will not be charged to their full capacity. The result is a reduced cycle life for the battery as compared to its individual cells.
A redox shuttle offers the best approach to cell overcharge protection. In this scheme, a material with an appropriate oxidation potential is dissolved in the electrolyte where it remains unreactive until the cell is charged fully. At a potential slightly above the cell charge limit (upper cutoff voltage), the redox shuttle is activated by its electrochemical conversion. The cell potential during overcharge is fixed at the oxidation potential of the redox shuttle. This process is supported by diffusion of the oxidized products to the anode where they recombine to form the starting material. Once the reformed material diffuses back to the cathode, it is oxidized and the cathode potential is maintained indefinitely at the oxidation potential of the redox reagent, until the time that charging is terminated.
Necessary properties of a redox shuttle include: high solubility in the electrolyte; an oxidation potential slightly higher than the normal charge limit of the cell but lower than the oxidation potential of the electrolyte; the ability of the oxidized form to be reduced at the anode without side reactions; and chemical stability in the cell of both the oxidized and reduced forms of the shuttle reagent.
Accordingly, an object of this invention is the use of redox reagents to provide a means of chemical overcharge protection to secondary non-aqueous liquid and polymer electrolyte cells with lithium or lithium-ion anodes.